Rare earth elements (a generic name of scandium, yttrium and 15 kinds of lanthanide elements) are used in large quantities as a constitutive element of important materials such as a magnet, an abrasive for hard disk, a hydrogen secondary battery, a catalyst for exhaust gas treatment, an MRI imaging agent and the like. These elements are separated and recovered from the above-mentioned materials as well as separated and purified from monazite, bastnaesite or zenotime which is a mineral ore.
As conventional separation processes of a rare earth element, (1) an ion exchange process, (2) a precipitation process and (3) a solvent extraction process are known. Among them, because the solvent extraction process of (3) enables a continuous separation of large quantities of a rare earth element, it can be considered as the most effective separation process among them.
In the above-mentioned solvent extraction process, by contacting an aqueous solution containing a rare earth element with an extraction agent or an aqueous solution containing an extraction agent and transferring the element from an aqueous layer to an organic layer, the rare earth element can be separated. As the extraction agent, acidic compounds such as dialkylphosphonic acids and carboxylic acids described in p. 251 of “Rare Metal Jiten” (edited by Masao Douyama, issued by Fujitec Corporation, 1991), and neutral compounds such as phosphate esters described in p. 21 of “Kidoruigenso No Kagaku” (written by N. E. Topp, translated by Jiro Shiokawa and Ginya Adachi, issued by Kagaku-Dojin Publishing Co. Inc., 1974) are generally widely known to the public and utilized.
However, by being contacted with an aqueous solution containing a rare earth element, the above-mentioned acidic compounds release hydrogen ions into the aqueous solution and significantly change the characteristics of the aqueous solution, particularly the concentration of hydrogen ions in the aqueous solution. This comprises a problem in that the transfer efficiency of a rare earth element from an aqueous layer to an organic layer is significantly lowered.
On the other hand, the above-mentioned neutral compounds known to date comprises a problem in that the transfer efficiency of a rare earth element from an aqueous layer to an organic layer is intrinsically low.